Fabrication of high density multilayer interconnect printed circuit boards

ABSTRACT

High density built-up multilayer printed circuit boards are produced by constructing microvias with photoimageable dielectric materials. A photosensitive dielectric composition on a conductive foil is laminated to conductive lines on a core. After imaging the foil and imaging and curing the photosensitive dielectric composition, vias are formed to the conductive lines. Thereafter the conductive lines are connected through the vias to the conductive foil, and then the conductive foil is patterned.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of copending provisional application60/044,069 filed Apr. 16, 1997.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention pertains to the production of printed circuit boards. Moreparticularly, the invention concerns the production of high densitybuilt-up multilayer circuit boards by constructing microvias withphotoimageable dielectric materials.

2. Description of the Prior Art

As the need for faster, smaller, less expensive integrated circuitproducts continues grow, the ability to wire-bond reaches the limits ofthe available technology and chips must be mounted using a flip-chipapproach and solder bumps. This leads to a direct chip attachmentpackage. The requirement to fan-out the high number of I/O's from theunderside of the chip places increasing demands on utilization of theprinted circuit board area. Plated-through-holes use too much space andblock routing channels. This drives the need for a high density packagewith a significant number of interconnections on the outer surface ofthe board as well as for increasing use of blind microvias.

Resin coated copper (RCC) has been used in the past to economicallyfabricate high density built-up multilayer circuit boards. Currentlymicrovias in such circuit boards fabricated with RCC are produced by twomethods, including plasma etching and laser drilling. As such, onlyprinted circuit fabricators with access to plasma etching or laserdrilling equipment can provide these advanced, blind-via boards. Thehigh cost of the plasma and laser equipment hinders widespread adoptionof RCC technology. Furthermore, the technical disadvantages associatedwith the plasma etching and laser drilling techniques, such asundercutting due to isotropic etching of plasma, and low throughput dueto sequential drilling by laser, also limit large scalecommercialization of RCC based high density multilayer circuit boards.

Alternatively, photovia processes, which use photoimageable dielectricmaterials to fabricate builtup multilayer printed circuit boards havebeen developed. In theses processes, photodielectrics are coated on apatterned core and photoimaged to define via holes. The via holes alongwith the surface of the dielectric layer are then plated with copper.U.S. Pat. No. 5,354,593 sequentially laminates and photoimages twophotodielectrics onto a conductive core to define via holes and thencopper plates the via holes. U.S. Pat. No. 5,451,721 produces amultilayer printed circuit board by applying a photosensitive resinlayer onto a core having a metal line on its surface. After imaging toform via holes, the resin layer is deposited with a copper layer byelectroless plating techniques. U.S. Pat. No. 5,334,487 produces apattered layer on a substrate by applying and exposing differentphotosensitive compositions on opposite sides of a copper foil. One sideis developed and the copper etched, followed by developing the otherside and metallization of through holes.

The foregoing photovia technologies allow for fabrication of highdensity interconnection printed circuit boards with conventionalequipment but they suffer from similar drawbacks such as difficultcopper plating processes and poor resin-to-copper adhesion. Theseproblems usually lead to poor reliability of the circuit boards. Theseproblems are solved by the present invention whereby a photosensitivedielectric composition on a conductive foil is laminated to conductivelines on a substrate. After imaging the foil, and imaging and curing thephotosensitive dielectric composition, vias are formed to the conductivelines. Thereafter the conductive lines are connected through the vias tothe conductive foil, and then the conductive foil is patterned.

SUMMARY OF THE INVENTION

The invention provides a process for producing a printed circuit boardwhich comprises:

(a) attaching a photosensitive element onto a pattern of conductivelines on the surface of a substrate; which photosensitive elementcomprises a negative working photosensitive dielectric composition on asurface of a conductive foil, such that the photosensitive dielectriccomposition is positioned on the conductive lines;

(b) applying a layer of a photoresist onto an opposite surface of saidfoil;

(c) imagewise exposing the photoresist to actinic radiation anddeveloping the photoresist to thereby form imagewise removed andimagewise nonremoved portions of the photoresist such that the imagewiseremoved portions are above at least some conductive lines;

(d) removing the portions of the conductive foil underlying theimagewise removed portions of the photoresist without removing theunderlying photosensitive dielectric composition;

(e) imagewise exposing a portion of the photosensitive dielectriccomposition to actinic radiation through the removed portions of theconductive foil; developing the photosensitive dielectric composition tothereby form imagewise removed and imagewise nonremoved portions of thephotosensitive dielectric composition such that the imagewise removedportions form vias to the conductive lines;

(f) curing the nonremoved portions of the photosensitive dielectriccomposition;

(g) electrically connecting the conductive lines through the vias to apart of the conductive foil; and

(h) patterning the conductive foil to thereby produce a pattern ofconductive foil lines.

The invention also provides a process for producing a printed circuitboard which comprises applying a layer of a negative workingphotosensitive dielectric composition onto a surface of a conductivefoil thereby forming a photosensitive element and then following steps(a) through (h) above.

The invention further provides process for producing a printed circuitboard which comprises:

(a) attaching a photosensitive element onto a pattern of conductivelines on the surface of a substrate; which photosensitive elementcomprises a negative working photosensitive dielectric composition on asurface of a conductive foil, such that the photosensitive dielectriccomposition is positioned on the conductive lines;

(b) removing the conductive foil;

(c) imagewise exposing a portion of the photosensitive dielectriccomposition to actinic radiation and developing the dielectriccomposition to thereby form imagewise removed and imagewise nonremovedportions of the dielectric composition such that the imagewise removedportions are above at least some conductive lines thus forming vias tothe conductive lines;

(d) curing the nonremoved portions of the photosensitive dielectriccomposition;

(e) simultaneously forming an electrically conductive layer on thenonremoved portions of the dielectric composition and electricallyconnecting the conductive lines through the vias to the electricallyconductive layer; and

(f) patterning the electrically conductive layer to thereby produce apattern of conductive lines.

The invention still further provides a process for producing a printedcircuit board which comprises applying a layer of a negative workingphotosensitive dielectric composition onto a surface of a conductivefoil thereby forming a photosensitive element and then following steps(a) through (f) in the preceding paragraph.

By the process of the invention microvias are produced by using negativeacting photosensitive resin coated metals such as copper. This productand process allows a substantial reduction in the cost of printedcircuit board fabrication process as compared to plasma or laserdrilling techniques. The photo microvia technology also avoids thetechnical barriers associated with the plasma and laser drilling methodssuch as undercutting due to isotropic etching of plasma and lowthroughput due to sequential drilling by laser. As compared with theexisting photovia technologies, this invention permits easy copperplating and better copper-to-resin adhesion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic view of a photosensitive element according tothe invention laminated to a printed substrate having metallic lines.

FIG. 2 shows a post lamination view of a photosensitive elementpositioned on the substrate and after a photoresist is applied to theopposite side of the photographic element.

FIG. 3 shows the structure of FIG. 3 after photoresist removal and foilimaging

FIG. 4 shows the structure of FIG. 3 after an imaging of the dielectriccomposition.

FIG. 5 shows the structure of FIG. 4 after removal of the nonimage areasof the dielectric composition to form vias.

FIG. 6 shows the structure of FIG. 5 after plating the vias andproviding an electrical connection between the conductive lines and theconductive foil.

FIG. 7 shows another embodiment of the invention where thephotosensitive element according to the invention laminated to a printedsubstrate having metallic lines and the entire conductive foil removed.

FIG. 8 shows the structure of FIG. 7 after an imaging of the dielectriccomposition.

FIG. 9 shows the structure of FIG. 8 after removal of the nonimage areasof the dielectric composition to form vias.

FIG. 10 the structure of FIG. 9 after plating the vias the top of thedielectric composition to provide a conductive top surface.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

One performs a first process embodiment of the invention by employing aphotosensitive element which comprises a negative working photosensitivedielectric composition applied on a surface of a conductive foil. Thenegative working photosensitive dielectric composition is one which issuitable for use as a permanent dielectric in electronic circuits.

Suitable conductive foils include copper, copper alloys, aluminum,aluminum alloy, and the like, however, copper foils are most preferred.

Suitable negative working photosensitive dielectric compositions includephotopolymerizable compositions which comprise at least onephotopolymerizable compounds contain at least two olefinicallyunsaturated double bonds, such as acrylates plus a free radicalphotoinitiator. Other negative working photoimageable compositions maybe produced by admixing a photoacid generator capable of generating anacid upon exposure to actinic radiation with a polymer precursor, suchas an epoxy precursor, which fonms polymers upon contact with thegenerated acid together with an optional, but preferred, organic acidanhydride monomer or polymer and an optional but preferredphenol-containing monomer or polymer. A combination of both types ofworking photosensitive dielectric compositions is also within thepurview of the invention.

Suitable photopolymerizable compounds containing at least twoolefinically unsaturated double bonds are well known in the art.Suitable for use as polymerizable compounds are ethers, esters andpartial esters of acrylic and methacrylic acid and aromatic andaliphatic polyols containing preferably 2 to 30 carbon atoms, orcycloaliphatic polyols containing preferably 5 or 6 ring carbon atoms.These polyols can also be modified with epoxides such as ethylene oxideor propylene oxide. The partial esters and esters of polyoxyalkyleneglycols are also suitable. Examples are ethylene glycol dimethacrylate,diethylene glycol dimethacrylate triethylene glycol dimethacrylate,tetraethylene glycol dimethacrylate, polyethylene glycol dimethacrylateshaving an average molecular weight in the range from 200 to 2000,ethylene glycol diacrylate, diethylene glycol diacrylate, triethyleneglycol diacrylate, tetraethylene glycol diacrylate, polyethylene glycoldiacrylates having an average molecular weight in the range from 200 to2000, trimethylolpropane ethoxylate trimethacrylate, trimethylolpropanepolyethoxylate trimethacrylates having an average molecular weight inthe range from 500 to 1500, trimethylolpropane ethoxylate triacrylateshaving an average molecular weight in the range from 500 to 1500,pentaerythritol diacrylate, pentaerythritol triacrylate, pentaerythritoltetraacrylate, dipentaerythritol diacrylate, dipentaerythritoltriacrylate, dipentaerythritol tetraacrylate, dipentaerythritolpentaacrylate, dipentaerythritol hexaacrylate, tripentaerythritoloctaacrylate, pentaerythritol dimethacrylate, pentaerythritoltrimethacrylate, dipentaerythritol dimethacrylate, dipentaerythritoltetramethacrylate, tripentaerythritol octamethylacrylate, 1,3-butanedioldimethacrylate, sorbitol triacrylate, sorbitol tetraacrylate, sorbitoltetramethacrylate, sorbitol pentaacrylate, sorbitol hexaacrylate,oligoester acrylates, oligoester methacrylates, glycerol di- andtriacrylate, 1,4-cyclohexane diacrylate, bisacrylates andbismethacrylates of polyethylene glycols having an average molecularweight from 100 to 1500, ethylene glycol diallyl ether,1,1,1-trimethylolpropane triallyl ether, pentaerythritol triallyl ether,diallyl succinates and diallyl adipates or mixtures of the abovecompounds. Preferred multifunctional acrylate oligomers include, but arenot limited to acrylated epoxies, acrylated polyurethanes, and acrylatedpolyesters. The photopolymerizable compound is present in an amountsufficient to photopolymerize upon exposure to sufficient actinicradiation. In the preferred embodiment, the multifunctionalphotopolymerizable compound is present in the overall composition in anamount of from about 1% to about 80% by weight, preferably from about20% to about 70% based on the non-solvent parts of the overall radiationsensitive composition.

When photopolymerizable compositions are used they contain at least onefree radical generating component which photolytically generates freeradicals. Examples of free radical generating components includephotoinitiators which themselves photolytically generate free radicalsby a fragmentation or Norrish type 1 mechanism. These latter have acarbon-carbonyl bond capable of cleavage at such bond to form tworadicals, at least one of which is capable of photoinitiation. Suitableinitiators include aromatic ketones such as benzophenone, acrylatedbenzophenone, 2-thylanthraquinone, phenanthraquinone,2-tert-butylanthraquinone, 1,2-benzanthraquinone, 2,3-benzanthraquinone,2,3-dichloronaphthoquinone, benzyl dimethyl ketal and other aromaticketones, e.g. benzoin, benzoin ethers such as benzoin methyl ether,benzoin ethyl ether, benzoin isobutyl ether and benzoin phenyl ether,methyl benzoin, ethyl benzoin and other benzoins;diphenyl-2,4,6-trimethyl benzoylphosphine oxide; andbis(pentafluorophenyl)titanocene. The free radical generating componentmay comprise a combination of radical generating initiators whichgenerate free radicals by a Norrish type 1 mechanism and a spectralsensitizer. Such a combination includes2-methyl-1-4′-(methylthio]-2-morpholinopropiophenone available from CibaGeigy as Irgacure 907 in combination with ethyl Michler's ketone (EMK)which is 4,4′-bisdiethylaminobenzophenone; Irgacure 907 in combinationwith 2-isopropylthioxanthanone (ITX); benzophenone in combination withEMK; benzophenone in combination with ITX; 2-benzyl-2-N,N-dimethylamino-1-(4-morpholinophenyl)-1-butanone which is availablefrom Ciba-Geigy as Irgacure 369 in combination with EMK; Irgacure 369 incombination with ITX. In such cases, it is preferred that the weightratio of radical generating photoinitiator and spectral sensitizerranges from about 5:1 to about 1:5. Other radical generators useful forthis invention non-exclusively include triazines, such as chlorineradical generators such as2-substituted-4,6-bis(trihalomethyl)-1,3,5-triazines. The foregoingsubstitution is with a chromophore group that imparts spectralsensitivity to the triazine to a portion of the electromagneticradiation spectrum. Non-exclusive examples of these radical generatorsinclude2-(4-methoxynaphth-1-yl)-4,6-bis(trichloromethyl)-1,3,5,-triazine;2-(4-methylthiophenyl)-4,6-bis(trichloromethyl)-1,3,5,-triazine;2-(4-methoxystyryl)-4,6-bis(trichloromethyl)-1,3,5-triazine;2-(4-diethylaminophenyl-1,3-butadienyl)-4,6-bis(trichloromethyl)-1,3,5-triazine,among others. Also useful for the invention are Norrish type IImechanism compounds such as combinations of thioxanthones such as ITXand a source of abstractable hydrogen such as triethanolamine. The freeradical generating component is present in an amount sufficient toeffect photopolymerization of the photopolymerizable compound uponexposure to sufficient actinic radiation. The photoinitiator maycomprise from about 1% to about 50% of the non-solvent parts of theoverall composition, or more preferably from about 2% to about 40% andmost preferably from about 5% to about 25%.

The negative working photoimageable compositions may also be produced byadmixing a photoacid generator capable of generating an acid uponexposure to actinic radiation, with polymer precursors, such as epoxyprecursors, which form polymers upon contact with the generated acid.The photoacid generator that may be used herein is one which generatesan acid upon exposure to actinic radiation such as ultravioletradiation. Photoacid generators are known in the photoimaging art andinclude, but are not limited to, onium compounds such as arylderivatives of sulfonium, iodonium and diazonium salts, and organiccompounds with photolabile halogen atoms. Preferred photoacid generatorsinclude tryarylsulfonium and diaryliodonium salts withhexafluorophosphate, hexafluoroantimonate, hexafluoroarsenate, andtetrafluoroborate anions. Non-limiting examples of suitable iodoniumsalts are salts of diphenyliodonium, dinaphthyliodonium,di(4-chlorophenyl)iodonium, tolyl(dodecylphenyl)iodonium,naphthylphenyliodonium, 4-(tri-fluoromethylphenyl)phenyliodonium,4-ethylphenyl-phenyliodonium, di(4-acetylphenyl)iodonium,tolylphenyliodonium, 4-butoxyphenylphenyliodonium,di(4-phenylphenyl)iodonium, and the like. Di-phenyliodonium salts arepreferred. Non-limiting examples of suitable sulfonium salts are saltsof triphenylsufonium, dimethylphenylsulfonium, tritolylsulfonium,di(methoxynaphthyl)methylsulfonium, dimethylnaphthylsulfonium,4-butoxyphenyldiphenylsulfonium, and 4-acetoxyphenyldiphenylsulfonium.Tri-phenylsulfonium salts are preferred. Organic compounds withphotolabile halogen atoms include alpha-halo-p-nitrotoluenes,alpha-halomethyl-s-triazines, carbon tetrabromide, and the like. Theseacid generators may be used singly or in combination of two or morethereof The photoacid generator component is preferably present in anamount of from about 0.05% to about 20% of the total weight of thenonsolvent parts of the composition, more preferably from about 0.2% toabout 10%, and most preferably from about 0.5% to about 5% by weight ofthe nonsolvent parts of the composition.

Suitable polymer precursors include epoxy precursors, for example, thediglycidyl ethers of resorcinol, catechol, hydroquinone, biphenol,bisphenol A, bisphenol F, bisphenol K, tetrabromobisphenol A,phenol-formaldehyde novolac resins, alkyl substitutedphenol-formaldehyde resins, phenol-hydroxybenzaldehyde resins,cresol-hydroxybenzaldehyde resins, dicyclopenadiene-phenol resins,dicyclopentadiene-substituted phenol resins tetramethylbiphenol,tetramethyl-tetrabromobiphenol, any combination thereof and the like.Also suitable are the alkylene oxide adducts of compounds of more thanone aromatic hydroxyl group per molecule such as the ethylene oxide,propylene oxide, or butylene oxide adducts of dihydroxy phenols,biphenols, bisphenols, halogenated bisphenols, alkylated bisphenols,trisphenols, phenol-aldehyde novolac resins, halogenated phenol-aldehydenovolac resins, alkylated phenolaldehyde novolac resins,phenol-hydroxybenzaldehyde resins, cresol-hydroxybenzaldehyde resins,any combination thereof and the like. Also suitable are the glycidylethers of compounds having an average of more than one aliphatichydroxyl group per molecule such as aliphatic polyols and polyetherpolyols. Non-limiting examples include polyglycidyl ethers ofpolyethylene glycols, polypropylene glycols, glycerol, polyglycerols,trimethylolpropane, butanediol, sorbitol, pentaerythritol, andcombinations thereof. The epoxy precursor component is preferablypresent in an amount of from about 10% to about 90%, more preferablyfrom about 20% to about 80% and most preferably from about 35% to about65% by weight of the nonsolvent parts of the composition. Optionally thenegative working photosensitive dielectric compositions can comprise amixture of both an aaylate and an epoxy type composition as describedabove. The composition then preferably contains an optional organic acidanhydride monomer or polymer curing agent component. Nonlimitingexamples of suitable anhydrides include styrene-maleic anhydride,styrene-alkyl methacrylate-itaconic anhydride, methyl methacrylate-butylacrylate-itaconic anhydride, butyl acrylate-styrene-maleic anhydride,and the like. Preferred are styrene-maleic anhydride polymers withstyrene to maleic anhydride molar ratio of from about 1:1 to about 3:1.Also suitable are dodecenyl succinic anhydride, trimellitic anhydride,chloroendic anhydride, phthalic anhydride, methylhexahydrophthalicanhydride, 1-methyl tetrahydrophthalic anhydride, hexahydrophthalicanhydride, methylnadic anhydride, methylbutenyltetrahydrophthalicanhydride, benzophenone terracarboxylic dianhydride,methylcyclohexenedicarboxylic anhydride. These acid anhydrides may beused singly or in combination of two or more thereof. This anhydridecomponent is preferably present in the composition in an amount of fromabout 0.5% to about 90%, more preferably from about 1% to about 80% andmost preferably from about 2% to about 60% by weight of the nonsolventpats of the composition The composition then contains an optionalaromatic hydroxyl containing compounds such as a phenolic monomer orpolymer or mixture thereof. Suitable aromatic hydroxyl containingcompounds which can be employed herein include, for example, compoundshaving an average of more than one phenolic hydroxyl group per molecule.Suitable such compounds include, for example, dihydroxy phenols,bi-phenols, bisphenols, halogenated bisphenols, alkylated bisphenols,trisphenols, phenol-aldehyde resins, halogenated phenol-aldehyde novolacresins, alkylated phenol-aldehyde novolac resins,phenol-hydroxybenzaldehyde resins, alkylated phenol-hydroxybenzaldehyderesins, the ethylene oxide, propylene oxide, or butylene oxide adductsof dihydroxy phenols, biphenols, bisphenols, halogenated bisphenols,alkylated bisphenols, trisphenols, phenol-aldehyde novolac resins,halogenated phenol-aldehyde novolac resins, alkylated phenol-aldehydenovolac resins, cresol-aldehyde novolac resins,phenol-hydroxybenzaldehyde resins, cresol-hydroxybenzaldehyde resins,vinyl phenol polymers, any combination thereof and the like. When phenolcontaining compounds or polymers are used, it is preferably present inan amount of from about 0.5% to about 90%, more preferably from about 1%to about 80%, and most preferably from about 2% to about 60% based onthe weight of the nonsolvent parts of the composition

Optionally, the photosensitive dielectric composition may comprises acuring catalyst such as a thermal curing catalyst, for example, tertiaryamines, imidazoles phosphines. The thermal curing catalyst may bepresent in an amount of from about 0.01% to about 10%, more preferablyfrom about 0.02% to about 5% and most preferably from about 0.05% toabout 2% by weight of the nonsolvent parts of the photosensitivedielectric composition.

The components of the photodielectric composition may be mixed in anysuitable medium solvent and coated onto the conductive foil by anyconvenient means. Solvents which can be used in preparing thephotopolymerizable composition of this invention include alcohols suchas methanol, ethanol, propanol and butanol; ketones such as acetone,methyl ethyl ketone, methyl isobutyl ketone, cyclohexanone, diisobutylketone, etc., esters such as ethyl acetate, butyl acetate, amyl acetate,methyl formate, ethyl propionate, dimethyl phthalate, ethyl benzoate andmethyl Cellosolve acetate; aromatic hydrocarbons such as toluene,xylene, benzene, ethylbenzene; halogenated hydrocarbons such as carbontetrachloride, trichloroethylene, chloroform, 1,1,1-trichloroethane,1,2-dichloroethane, monochlorobenzene, chloronaphthalene; ethers such astetrahydrofuran, diethyl ethers ethylene glycol monoethyl ether acetate,ethylene glycol monomethyl ether, etc., dimethylformamide, dimethylsulfoxide, etc., and mixtures thereof. The most preferred solvents areethyleneglycol monomethylether, ethyleneglycol monoethylether anddimethyl formamide which dissolve the other components of thephotographic coating. A suitable amount of the solvent which can beemployed in the photopolymerizable composition of this invention rangesfrom about 2050% to about 1,0000%, preferably 50% to 5000%, by weight ofthe total non-solvent parts of the composition. The preparedphotodielectric composition is then coated on the foil substrate by wellknown techniques such as but not limited to spin coating, slot diecoating, extruding, Meyer rod drawing, blade drawing, screen coating,curtain coating, dip coating, or spray coating. Once the photodielectriccomposition coating is applied to the substrate, the solvents areevaporated to yield a dry coating weight of from about 20 to about 200g/m², more preferably from about 40 to about 150 g/m², and mostpreferably from about 50 to about 100 g/m². A protective film mayoptionally be attached to the photodielectric composition until it isready for use. Suitable photodielectric resins are commerciallyavailable under the trade name of XP-9500 from Shipley, and Probelec®XB-7081 from Ciba Specialty Chemicals.

As seen in FIG. 1, the photosensitive element comprising the conductivefoil 2 and photosensitive dielectric composition 4 is then attached ontoa pattern of conductive lines 6 which is on the surface of a substrate8. Suitable substrates include those which are well known in the art forproducing printed circuit boards such as polyesters, polyimides,epoxies, teflon and silicon. Most preferably the substrate is aninsulating epoxy board. The pattern of conductive lines may be a metalsuch copper, copper alloys, aluminum, alloy, and the like, however,copper is most preferred. Within the context of the invention, the termmetal lines includes electrical bonding pads. These may be produced bywell known photolithographic and etching processes. Preferably thephotosensitive element is attached to the metal lines and the substrateby means of lamination. That is, the photosensitive element and thesubstrate are passed through the nip of a set of heated rollers or aheated press in an laminating device with the temperature at about fromabout 90° C. to about 150° C.

As seen in FIG. 2, one then applies a layer of a photoresist 10 onto anopposite surface of foil 2. Notice that the dielectric composition layer4 is now positioned both above and between the conductive lines 6. Thephotoresist may be positive working or negative working. Useful negativeworking photoresists include those compositions described above as beinguseful for the photosensitive dielectric composition. Suitable positiveworking photoresists are well known in the art and may comprise apositive working o-quinone diazide radiation sensitizer. The o-quinonediazide sensitizers include the o-quinone-4-or-5-sulfonyl-diazidesdisclosed in U.S. Pat. Nos. 2,797,213; 3,106,465; 3,148,983; 3,130,047;3,201,329; 3,785,825; and 3,802,885. When o-quinone diazides are used,preferred binding resins include a water insoluble, aqueous alkalinesoluble or swellable binding resin, which is preferably a novolak. Theproduction of novolak resins is well known in the art. A procedure fortheir manufacture is described in Chemistry and Application of PhenolicResins, Knop A and Scheib, W.; Springer Verlag, N.Y., 1979 in Chapter 4which is incorporated herein by reference. Suitable novolak resins arewater insoluble, aqueous alkali soluble resins having a preferredmolecular weight in the range of from about 6,000 to about 14,000, ormore preferably from about 8,000 to about 12,000. The amount of thesensitizers and binder can be experimentally varied by one skilled inthe art depending on the desired product characteristics. The componentsare blended with a suitable solvent, such as those listed above, coatedonto the conductive foil and dried. Suitable photoresist compositionsare described in U.S. Pat. No. 4,588,670. Alternatively, the photoresistmay be a dry film photoresist such as MacDermid Aquamer dry filmphotoresist. The photoresist is then imagewise exposed to actinicradiation. Such may either be through a photomask or by laser exposure.Exposed may be to ultraviolet radiation, such as in the 300 to 550nanometer range through a photographic mask or computer directed laserpattern and developed. Suitable uv light sources are carbon arc lamps,xenon arc lamps, mercury vapor lamps which may be doped with metalhalides (metal halide lamps), fluorescent lamps, argon filament lamps,electronic flash lamps and photographic floodlight lamps. Exposure isconducted to provide sufficient actinic energy to the element to permita photochemical change in the image areas where the light sensitivecomposition is exposed through a mask and yet substantially prevent anyphotochemical change in the nonimage areas. The exposed photoresist isthen developed to thereby form imagewise removed and imagewisenonremoved portions of the photoresist such that the imagewise removedportions are above at least some conductive lines. Typical developercompositions can be alkaline or neutral in nature and have a pH range offrom about 5 to about 12. Developers are preferably formed from aqueoussolutions of phosphates, silicates or metabisulfites. Suchnon-exclusively include mono-, di- and tri-alkali metal phosphate,sodium silicate, alkali metal metasilicate and alkali metabisulfite.Alkali metal hydroxides may also be used although these are notpreferred. The developers may also contain art recognized surfactants,buffers, solvents and other ingredients.

Next one removes the portions of the conductive foil underlying theimagewise removed portions of the photoresist without removing theunderlying photosensitive dielectric composition. The conductive foilportion to be removed may be so removed by such known techniques asetching and laser ablation. FIG. 3 shows the conductive foil withimagewise removed portion after removal of the balance of thephotoresist layer.

Next one imagewise exposes a portion of the photosensitive dielectriccomposition to actinic radiation through the removed portions of theconductive foil in a manner described above. The exposed portions 12 ofthe dielectric layer are seen in FIG. 4. The conductive foil mayoptionally be used as a conformal mask, however, in the preferredembodiment, a second pattern is used, either a different mask or adifferent laser exposure pattern, to expose a part of the photosensitivedielectric composition which is revealed through the removed conductivefoil portions. Thereafter the photosensitive dielectric composition isdeveloped in a manner similar to that described above to thereby formimagewise removed and imagewise nonremoved portions of thephotosensitive dielectric composition such that the imagewise removedportions form vias 14 to the conductive lines 6 as seen in FIG. 5.

The nonremoved portions of the photosensitive dielectric composition arethen cured, preferably thermally cured. Curing may be effected byheating at temperatures of from about 90° C. to about 250° C. for fromabout 10 minutes to about 120 minutes.

One then electrically connects the conductive lines through the vias toa part of the conductive foil. This is preferably done by plating ametal 16 through the vias from the conductive lines 6 to a part of theconductive foil 2 as seen in FIG. 6. Such may be done by performing anelectroless metal plating through the vias from the conductive lines toa part of the conductive foil, optionally followed by performing a metalelectroplating step, each of which steps are well known in the art.Optionally the vias may be filled by a conductive paste such as U-300available from Epoxy Technology, Inc. or organo-metallic compounds suchOrmat available from Ormat Inc. of Carlstadt, Calif. Thereafter theconductive foil is preferably patterned by means well known in the artto thereby produce a pattern of conductive foil lines. Optionally theprocess steps may be repeated one or more times by attaching anotherphotosensitive element onto the previously pattered conductive foillines resulting from the process as described above to form amutilayered structure. Optionally the entire process may be conductedone or more time on both sides of the substrate to provide a dual sidedprinted circuit board.

In another embodiment of the invention, the above photosensitive elementis attached onto a pattern of conductive lines 6 on the surface of asubstrate 8 as previously described in FIG. 1. Thereafter, as seen inFIG. 7, the entire conductive foil 2 is removed, such as by etching orlaser ablation techniques leaving only the photosensitive dielectriccomposition 4 on and between the conductive lines 6 on the surface of asubstrate 8. This preferably imparts a microroughened, matte surface tothe photosensitive dielectric composition for better copper adhesion tothe dielectric layer. As seen in FIGS. 8 and 9, the photosensitivedielectric composition 4 is then imagewise exposed to actinic radiationand developed to thereby form imagewise removed and imagewise nonremovedportions of the dielectric composition such that the imagewise removedportions are above at least some conductive lines thus forming vias 14to the conductive lines 6. The nonremoved portions of the photosensitivedielectric composition are then cured. Then, as seen in FIG. 10, anelectrically conductive layer 18, such as copper, is formed on thedielectric composition, preferably by plating, and forms an electricalconnection from the conductive lines 6 through the vias 16 to theelectrically conductive layer 18. Thereafter the electrically conductivelayer 18 is patterned by means well known in the art to produce apattern of conductive lines from the electrically conductive layermaterial.

The following non-limiting examples serve to illustrate the invention.

EXAMPLE 1

A photodielectric resin available from Shipley under the trade name ofXP-9500 is coated on a half-ounce copper foil to form an approximately 2mil thick resin coated copper foil. After coating, the coated foil isprotected with a polyester release film until it is used to make acircuit. The photodielectric dry film backed by copper foil is laminatedto a circuitized inner layer board by a hot roll laminator with the rolltemperature at about 120° C. In the same step an additional layer ofMacDermid Aqua Mer dry film photoresist is laminated on top of thecopper foil. The ensemble is allowed to cool and the dry filmphotoresist is exposed to UV light through an artwork with the desiredfeatures. The artwork is clear except where it is desired to createphoto-vias through the resin to the next layer of copper circuitry. Themask has dark areas (typically circular) which are one to two mils widerin diameter than the desired photo-via opening. The dry film photoresistis developed, exposing the copper foil in the regions that were beneaththe circular dark regions of the mask during exposure. The copper foilis then etched away using a cupric chloride etchant at 50° C. Afterrinsing and drying, an annular ring type artwork is placed on top of theimaged foil. The annular ring mask is aligned such that the outer ringregisters with the perimeter of the etched hole in the copper foil. Thisartwork is primarily dark, with the annular rings being clear. Theannular ring is typically one to two mils wide. The panel is thenexposed to UV at about 1 J/cm² through the artwork. The exposed panel ispostbaked for 15 minutes at 90° C. and, after cooling down, the panel isthen immersed in and sprayed with 2% caustic aqueous solution at 50° C.to strip the dry film photoresist and develop the via holes of thephotosensitive layer down to the copper pads in the next layer. Theholes are then desmeared with a potassium permangnate desmear solution,cleaned with conventional cleaning solutions, rinsed, and dried. Thepanel is baked at 170° C. for 2 hours to cure the dielectric layer.Following cure, the panel is plated with conventional electroless copperplating solutions followed by a conventional electroplating of anadditional 1 mil of copper. Conductive vias between the two copperlayers are thus formed. The outer layer circuitry is fabricated withconventional print and etch processes. A photoresist dry film islaminated onto the copper plane and imaged through UV exposure anddeveloping. The panel is etched with conventional copper etchants. Afteretching the photoresist film is stripped with conventional strippers andthe panel cleaned with conventional cleaning solutions. The aboveprocess is repeated as many times as necessary to fabricate a printedwiring board having the desired number of build-up layers.

EXAMPLE 2

The photodielectric resin available from Ciba Specialty Chemicals underthe trade name of Probelec® XB-7081 is coated on a half-ounce copperfoil to form an approximately 2 mil resin coated copper. Thephotodielectric dry film backed by copper foil is laminated to acircuitized inner layer board by a vacuum press at about 90° C.MacDermid Aqua Mer dry film photoresist is laminated on top of thecopper foil with a hot roll laminator at 120° C. The ensemble is allowedto cool and the dry film photoresist is exposed to UV light through anartwork with the desired features. The artwork is clear except where itis desired to create photo-vias through the resin to the next layer ofcopper circuitry. The mask has dark areas (typically circular) which areone to two mils wider in diameter than the desired photo-via opening.The dry film photoresist is developed, exposing the copper foil in theregions that were beneath the circular dark regions of the mask duringexposure. The copper foil is etched away using a cupric chloride etchantat 50° C. The photoresist layer is stripped away using conventionalphotoresist strippers. After rinsing and drying, an annular ring typeartwork is placed on top of the imaged foil. The annular ring mask isaligned such that the outer ring registers with the perimeter of theetched hole in the copper foil. This artwork is primarily dark, with theannular rings being clear. The annular ring is typically one to two milswide. The panel is then exposed to UV at 1.5 J/cm² through the artwork.The exposed panel is postbaked for 15 minutes at 130° C. and, aftercooling down, the photodielectric layer is developed withgamma-butyrolactone to extend the via holes of the photodielectric layerdown to the copper pads in the next layer. The holes are then desmearedwith a potassium permangnate desmear solution, cleaned with conventionalcleaning solutions, rinsed, and dried. The panel is baked at 170° C. for2 hours to cure the dielectric layer. Following cure, the panel isplated with conventional electroless copper plating solutions followedby a conventional electroplating of an additional 1 mil of copper.Conductive vias between the two copper layers are thus formed. The outerlayer circuitry is fabricated with conventional print and etchprocesses. A photoresist dry film is laminated onto the copper plane andimaged through UV exposure and developing. The panel is etched withconventional copper etchants. After etching the photoresist film isstripped with conventional strippers and the panel cleaned withconventional cleaning solutions. The above process is repeated as manytimes as necessary to fabricate a printed wiring board having thedesired number of build-up layers.

EXAMPLE 3

The photodielectric resin available from Shipley under the trade name ofXP-9500 is coated on a half-ounce copper foil to form an approximately 3mil thick resin coated copper foil. After coating, the coated foil isprotected with a polyester release film until it is used to make acircuit. The photodielectric dry film backed by copper foil is laminatedto a circuitized inner layer board by a hot roll laminator with the rolltemperature at about 120° C. The copper foil is etched away using acupric chloride etchant at 50° C. After rinsing and drying, the panel isexposed to UV through an artwork with desired features at 1 J/cm². Thepanel is then exposed to UV at about 1 J/cm² through the artwork. Theexposed panel is postbaked for 15 minutes at 90° C. and, after coolingdown, the panel is then immersed in and sprayed with 2% caustic aqueoussolution to develop the via holes of the photosensitive layer down tothe copper pads in the next layer. The holes are then desmeared with apotassium permangnate desmear solution, cleaned with conventionalcleaning solutions, rinsed, and dried. The photosensitive layer is floodexposed at 1 J/cm² and cured at 150° C. for 1 hour. Following cure, thepanel is plated with conventional electroless copper solutions followedby an additional electro-plating of 1-2 mils of copper. Conductive viasbetween the two copper layers are thus formed. The outer layer circuitryis fabricated with conventional print and etch processes. A photoresistdry film is laminated onto the copper plane and imaged through UVexposure and developing. The copper layer is etched with conventionalcopper etchants. After etching the photoresist film is stripped withconventional strippers and the panel cleaned with conventional cleaningsolutions. The above process is repeated as many times as necessary tofabricate a printed wiring board having the desired number of build-uplayers. The board is finally finished with whatever additional layerssuch as solder mask, solder, electroless gold, etc. are desired.

From the above it can be seen that high density, built-up multilayerprinted circuit boards can be produced by constructing microvias withphotoimageable dielectric materials.

What is claimed is:
 1. A process for producing a printed circuit boardwhich comprises; (a) attaching a photosensitive element onto a patternof conductive lines on the surface of a substrate wherein there arespaces between the conductive lines; which photosensitive elementcomprises a negative working photosensitive dielectric composition on asurface of a conductive foil, such that the photosensitive dielectriccomposition is positioned on the conductive lines and in the spacesbetween the conductive lines; (b) applying a layer of a photoresist ontoan opposite surface of said foil; (c) imagewise exposing the photoresistto actinic radiation and developing the photoresist to thereby formimagewise removed and imagewise nonremoved portions of the photoresistsuch that the imagewise removed portions are above at least oneconductive line; (d) removing the portion of the conductive foilunderlying the imagewise removed portions of the photoresist withoutremoving the underlying photosensitive dielectric composition; (e)imagewise exposing a portion of the photosensitive dielectriccomposition to actinic radiation through the removed portions of theconductive foil; developing the photosensitive dielectric composition tothereby form imagewise removed and imagewise nonremoved portions of thephotosensitive dielectric composition such that the imagewise removedportion form vias to the conductive lines; (f) curing the nonremovedportions of the photosensitive dielectric composition; (g) electricallyconnecting the conductive lines though the vias to a part of theconductive foil; and (h) patterning the conductive foil to therebyproduce a pattern of conductive foil lines.
 2. The process of claim 1further comprising repeating steps (a) through (h) at least once byattaching another photosensitive element according to step (a) onto thepreviously patterned conductive foil lines resulting from step (h). 3.The process of claim 1 wherein step (g) comprises plating a metalthrough the vias from the conductive lines to a part of the conductivefoil.
 4. The process of claim 1 wherein step (g) comprises performing anelectroless metal plating through the vias from the conductive lines toa part of the conductive foil.
 5. The process of claim 1 wherein step(g) comprises performing an electroless metal plating through the viasfrom the conductive lines to a part of the conductive foil followed byperforming a metal electroplating step.
 6. The process of claim 1wherein step (g) comprises filling the vias with a conductive paste ororganometallic compound.
 7. The process of claim 1 wherein step (f) isconducted by heating at a temperature of from about 90° C. to about 250°C.
 8. The process of claim 1 wherein the foil comprises copper, copperalloys, aluminum or aluminum alloys.
 9. The process of claim 1 whereinthe conductive lines comprise copper, copper alloys, aluminum oraluminum alloys.
 10. The process of claim 1 wherein the substratecomprises an insulating material.
 11. The process of claim 1 wherein thephotoresist is a negative working photosensitive composition.
 12. Theprocess of claim 1 wherein the photoresist is a positive workingphotosensitive composition.
 13. The process of claim 1 wherein thephotoresist is exposed through a first photographic mask and thephotosensitive dielectric composition is exposed through a differentsecond photographic mask.
 14. A process for producing a printed wiringboard which comprises: (a) applying a layer of a negative workingphotosensitive dielectric composition onto a surface of a conductivefoil thereby forming a photosensitive element; (b) attaching theconductive foil via the photosensitive dielectric composition onto apattern of conductive lines on the surface of a substrate wherein thereare spaces between the conductive lines; (c) applying a layer of aphotoresist onto an opposite surface of said foil; (d) imagewiseexposing the photoresist to actinic radiation and developing thephotoresist to thereby form imagewise removed and imagewise nonremovedportions of the photoresist such that the imagewise removed portions areabove at least one conductive line; (e) removing the portions of theconductive foil underlying the imagewise removed portions of thephotoresist without removing the underlying photosensitive dielectriccompositions; (f) imagewise exposing a portion of the photosensitivedielectric composition to actinic radiation through the removed portionsof the conductive foil; developing the photosensitive dielectriccomposition to thereby form imagewise removed and imagewise nonremovedportions of the photosensitive dielectric composition such that theimagewise removed portions form vias to the conductive lines; (g) curingthe nonremoved portions of the photosensitive dielectric composition;(h) electrically connecting the conductive lines thought the vias to apart of the conductive foil; and (i) patterning the conductive foil tothereby produce a pattern of conductive foil lines.
 15. The process ofclaim 14 further comprising repeating steps (a) through (i) at leastonce by attaching another photosensitive element according to step (a)onto the previously patterned conductive foil lines resulting from step(i).
 16. A process for producing a printed circuit board whichcomprises; (a) attaching a photosensitive element onto a pattern ofconductive lines on the surface of a substrate; which photosensitiveelement comprises a negative working photosensitive dielectriccomposition on a surface of a conductive foil, such that thephotosensitive dielectric composition is positioned on the conductivelines; (b) removing the conductive foil to expose the photosensitivedielectric composition wherein the exposed photosensitive dielectriccomposition has a microroughened surface; (c) imagewise exposing aportion of the microroughened photosensitive dielectric composition toactinic radiation and developing the dielectric composition to therebyform imagewise removed and imagewise nonremoved portions of themicroroughened dielectric composition such that the imagewise removedportions are above at least one conductive line; (d) curing thenonremoved portions of the microroughened photosensitive dielectriccompositions; (e) simultaneously forming an electrically conductivelayer on the microroughened surface of the nonremvoed protions of thedieletric composition and electrically connecting the conductive linesthrough the vias to the electrically conductive layer; and (f)patterning the electrically conductive layer to thereby produce apattern of conductive lines.
 17. The process of claim 16 furthercomprising repeating steps (a) through (f) at least once by attachinganother photosensitive element according to step (a) onto the previouslypattered conductive lines resulting from step (f).
 18. The process ofclaim 16 wherein step (e) comprises plating a metal through the viasfrom the conductive lines to the electrically conductive layer.
 19. Theprocess of claim 16 wherein step (e) comprises performing an electrolessmetal plating through the vias from the conductive lines followed byperforming a metal electroplating step from the conductive lines to theelectrically conductive layer.
 20. The process of claim 16 wherein thefoil comprises copper, copper alloys, aluminum or aluminum alloys. 21.The process of claim 16 wherein the conductive lines comprise copper,copper alloys, aluminum or aluminum alloys.
 22. The process of claim 16wherein the substrate comprises an insulating material.
 23. A processfor producing a printed circuit board which comprises (a) applying alayer of negative working photosensitive dielectric composition onto asurface of a conductive foil thereby forming a photoseneitive element;(b) attaching a conductive foil via the photosensitive dielectriccomposition onto a pattern of conductive lines on the surface of asubstrate; (c) removing the conductive foil to expose the photosensitivedielectric composition wherein the exposed photosensitive dielectriccomposition has a microroughened surface; (d) imagewise exposing aportion of the microroughened photosensitive dielectric composition toactinic radiation and developing the microroughened dielectriccomposition to thereby form imagewise removed and imagewise nonremovedportions of the microroughened dielectric composition such that theimagewise removed portions are above at least one conductive line thusforming vias to the conductive lines; (e) curing the nonremoved portionsof the microroughened photosensitive dielectric composition; (f)simultaneously forming an electrically conductive layer on themicroroughened surface of the nonremoved portions of the dielectriccomposition and electrically connecting the conductive lines though thevias to the electrically conductive layer; and (g) patterning theelectrically conductive layer to thereby produce a pattern of conductivelines.
 24. The process of claim 23 further comprising repeating steps(a) through (g) at least once by attaching another photosensitiveelement according to step (a) onto the previously pattered conductivefoil lines resulting from step (g).